A reciprocating engine, a piston engine, converts heat to mechanical energy. One or more pistons convert pressure into a rotating motion. A fuel-air mixture is introduced and ignited inside a cylinder, which houses a piston. Upon ignition, the hot gases expand, pushing the piston to the bottom of the cylinder. Piston recovery, return to the top of a cylinder, is achieved via either a flywheel or from the movement of additional pistons connected to a same shaft. The expanded gases are removed from the cylinder, exhausted by this stroke. The linear piston motion is converted to rotating motion via a connecting rod and a crankshaft. Increasing the number of combined pistons on a shaft yields a greater piston displacement volume and in turn a greater amount of power output.
The internal combustion reciprocating engine is widely used in almost every facet of the economy. Internal combustion reciprocating engines are used, for example, in automobiles, power generators, pumps, and power tools.
In some designs the piston may be powered in both directions in the cylinder, known as double acting.
Gasoline powered internal combustion reciprocating engines typically have efficiencies ranging from 20 to 25 percent, although theoretically internal combustion reciprocating engines can attain efficiencies up to 55%. Diesel engines, making use of higher compression ratios, yield higher efficiencies than internal combustion reciprocating engines but still do not approach 55 percent. Improvements in efficiency are always desirable for many significant reasons, which include economic gains, fuel conservation gains, and emission reductions per unit of fuel.